1. Field of the Invention
The present invention relates to a data acquisition circuit for acquiring data superposed on a video signal.
2. Description of Related Art
FIG. 11 is a block diagram showing a conventional data acquisition circuit.
Referring to FIG. 11, reference numeral 1 denotes an A/D converter for updating its reference potential so that the reference potential coincides with an analog potential of a video signal and outputting a digital value corresponding to the reference potential, reference numeral 2 a 0/1 judgment circuit for comparing the digital value outputted from the A/D converter 1 with a predetermined judgment reference value to convert the digital value into binary code and reference numeral 3 a memory for storing therein the binary-coded digital value (“0” or “1”) obtained by the 0/1 judgment circuit 2.
FIG. 12 is a block diagram showing an internal configuration of the A/D converter 1.
Referring to FIG. 12, reference numeral 11 denotes a D/A converter for updating the reference potential in accordance with a compared result obtained by a comparator 14 and outputting the updated reference potential, reference numerals 12 and 13 limiting resistors for limiting the variable voltage range of the reference potential and reference numeral 14 a comparator for comparing the reference potential whose variable voltage range is limited by the limiting resistors 12 and 13 with the analog potential of the video signal and outputting the digital value corresponding to the reference potential when the reference potential coincides with the analog potential.
Next, the operation of the conventional data acquisition circuit will be described.
First, the A/D converter 1 receives a video signal as an analog signal and successively updates its reference potential so that the reference potential coincides with the analog potential of the video signal. When the analog potential of the video signal coincides with the reference potential, the A/D converter 1 outputs a digital value corresponding to the reference potential.
Specifically, when the D/A converter 11 in the A/D converter 1 outputs the reference potential, the comparator 14 in the A/D converter 1 compares the reference potential whose variable voltage range is limited by the limiting resistors 12 and 13 with the analog potential of the video signal.
The comparator 14 outputs a control signal instructing a raising of the reference potential to the D/A converter 11 when the reference potential is lower than the analog potential of the video signal and outputs a control signal instructing a lowering of the reference potential to the D/A converter 11 when the reference potential is higher than the analog potential of the video signal.
Further, the comparator 14 outputs the digital value corresponding to the reference potential when the reference potential coincides with the analog potential of the video signal.
The reason why the limiting resistors 12 and 13 are connected to the output side of the D/A converter 11 is as follows.
In a case of 6-bit A/D converter 1, for example, when the variable voltage of the reference potential outputted from the D/A converter 11 ranges from 0 to 5 V, the resolution per 1 bit is 0.079 V and if the limiting resistors 12 and 13 are connected thereto to limit the variable voltage of the reference potential with the range from 1 to 3 V, the resolution per 1 bit comes to 0.032 V and the resolution per 1 bit increases by a factor of about 2.5.
When the A/D converter 1 outputs the digital value in the manner as described above, the 0/1 judgment circuit 2 compares the digital value with the predetermined judgment reference value.
Then, when the digital value is larger than the predetermined judgment reference value, the 0/1 judgment circuit 2 stores “1” into the memory 3. Otherwise, the 0/1 judgment circuit 2 stores “0” into the memory 3.
The conventional data acquisition circuit thus configured as above improves the resolution per 1 bit, but if a video signal having an analog potential out of the variable voltage range of the reference potential is inputted (for example, a video signal having an analog potential of 4 V is inputted when the variable voltage ranges from 1 to 3 V), the conventional circuit does not properly acquire data superposed on the video signal, or the like.